1. Field of the Invention
The present invention generally relates to a wireless relay system, a wireless relay apparatus, and a wireless relay method for wireless communication between a transmission apparatus and a reception apparatus via a single or a plurality of wireless relay apparatuses.
2. Description of the Related Art
Recently and continuing, a method of executing relay between a transmission apparatus and a reception apparatus by using the same frequency band with a wireless relay apparatus (=relay device) is being considered. In employing the wireless relay apparatus, it is important to control oscillation created by a coupling loop interference wave from a transmitter antenna to a receiver antenna. That is, a characteristic degraded by generation of oscillation is a problem.
Furthermore, since the relay is considered as a method that is mainly employed for broadcasting, controlling the coupling loop interference wave is relatively easy. Meanwhile, in a case where the wireless relay apparatus is situated in the same environment (not a steel tower or on top of a building) as a typical mobile terminal, such as in a mobile communication system, changes of channel of the coupling loop interference wave become more frequent in accordance with movement of the wireless relay apparatus or changes of the surrounding environment. There is a problem in which a highly advanced technology of canceling the coupling loop interference wave is required for swiftly corresponding to abrupt changes of the coupling loop interference wave.
In order to solve the problems of the conventional wireless relay apparatus, various methods are proposed.
For example, a method of canceling the coupling loop interference wave, which uses a pilot signal inserted by a transmitter apparatus, is proposed (See, for example, Japanese Laid-Open Patent Application Nos. 2003-174392 and 2003-298548). Japanese Laid-Open Patent Application No. 2003-174392 focuses on amplitude and phase being inserted into a predetermined pilot signal beforehand, in which an error signal is obtained by detecting frequency characteristics throughout the entire signal band by using the pilot signal, multipath components of transmission signals from a master station and other relay stations and a replica signal of the coupling loop interference wave are generated, and the replica signal is subtracted from the reception signal. Accordingly, even in a propagation environment where a large number of coupling loop interference waves exist or even in a propagation environment where multipath components of the main station transmission signal exist, the influences of both the coupling loop interference waves and the multipath components of the master station transmission signal can be eliminated.
According to Japanese Laid-Open Patent Application No. 2003-298548, a subtractor for subtracting duplications of coupling loop interference signals from input signals, an FIR filter for generating duplications of coupling loop interference signals, and a filter coefficient generation part for generating an FIR filter from the output of the subtractor are provided. The filter coefficient generation part performs a hard decision and remodulates a data carrier, calculates a transmission path characteristic by referring to the re-modulated data carrier, calculates cancellation residue from the transmission path characteristic, and updates the coefficient of the FIR filter based on the results of applying IFFT to the cancellation residue. Accordingly, a coupling loop canceller enables tracking performance to be improved and cancelable delay time to be expanded.
A configuration of a conventional wireless relay apparatus, which uses a coupling loop cancellation method by using a pilot signal inserted by a transmitter apparatus, is described below.
FIG. 15 is block diagram showing a configuration of a conventional wireless relay apparatus. As shown in FIG. 15, the wireless relay apparatus includes a subtractor 201 for subtracting a replica of a coupling loop interference signal from an input signal, an FIR filter 202 for generating the replica of the coupling loop interference signal, a coefficient update part 203 for generating a coefficient of the FIR filter 202, a pilot signal generation part 204 for generating a pilot signal with an amplitude and phase that are predefined, and an amplifier 205.
In the conventional wireless relay apparatus, the coefficient update part 203 obtains a characteristic of a transmission path, calculates a cancellation residue from the characteristic of the transmission path, and updates the coefficient of the FIR filter 202. Accordingly, tracking performance can be improved and cancelable delay time can be expanded.
As for other methods, there are a method of estimating a coupling loop interference by adding a pilot signal (a relatively long PN sequence with low electric power) to a relay signal of a wireless relay apparatus (see, for example, Japanese Laid-Open Patent Application No. 2001-186073); a method of estimating a coupling loop interference wave by transmitting a pilot signal from a wireless relay apparatus with a different frequency band (see, for example, Japanese Laid-Open Patent Application No. 8-331016); and a method of estimating a coupling loop interference wave by temporarily stopping relay (see, for example, Japanese Laid-Open Patent Application No. 2000-244382).
However, the coupling loop cancellation method disclosed in Japanese Laid-Open Patent Application No. 2003-174392 has a problem of increasing noise in the receiver.
In the coupling loop cancellation method disclosed in Japanese Laid-Open Patent Application No. 2003-298548, a part of noise voltage of a sub-carrier will be excessively amplified and transmitted since a transmission signal is amplified stronger as the signal received by a wireless relay apparatus becomes weaker. Accordingly, the bit error rate tends to increase.
Furthermore, the coupling loop cancellation method disclosed in Japanese Laid-Open Patent Application No. 2001-186073 has a problem of increasing the error rate in the receiver apparatus by applying a disturbance to a signal that is to be originally transmitted.
Furthermore, in the coupling loop cancellation method disclosed in Japanese Laid-Open Patent Application No. 8-331016, since the estimated channel value relies on a frequency that is used, accuracy in estimating the channel of the coupling loop is liable to deteriorate, and preparation of a frequency band for estimating the coupling loop interference wave would be necessary.
Furthermore, in the coupling loop cancellation method disclosed in Japanese Laid-Open Patent Application No. 2000-244382, there is a problem that communication between a transmitter apparatus and a receiver apparatus is temporarily interrupted at a timing unexpected by the transmitter apparatus.